(1) Field of the Invention
The present invention relates to n-type thermoelectric materials to be used for exhibiting so-called thermoelectric effects (direct energy conversion without relying upon moving parts) such as thermoelectric generation due to Seebeck effect or thermoelectric cooling due to Peltier effect.
(2) Related Art Statement
As the thermoelectric effects in the thermoelectric conversion such as thermoelectric generation and the thermoelectric cooling, the Seebeck effect, the Peltier effect, and the Thomson effect may be recited. The Seebeck effect is a phenomenon that when different kinds of conductors or a p-type conductor and a n-type conductor are joined together and one of the joined portions is set at a high temperature, whereas the other being at a low temperature, a thermally generated emf (electromotive force) occurs depending upon a temperature difference. The Peltier effect is a phenomenon that when different kinds of conductors or a p-type conductor and a n-type conductor are joined together and current is flown through them, heat is absorbed in one of the joined portions and heat is generated in the other. The Thomson effect is a phenomenon that one end of a uniform conductor or semiconductor is set at a high temperature, the other being at a low temperature, and DC current is passed therethrough along a temperature gradient, heat is absorbed into or emitted from the conductor or semiconductor depending upon a direction in which the current is flown. Such thermoelectric materials are suitably used in simplified direct energy converters having no moving parts susceptible of vibrations, noises, abrasion, etc., simplified structures, high reliability and long use life with easy maintenance. For example, such thermoelectric materials may be used for cases where DC power are obtained by the combustion of various foil fuels or where temperatures are controlled without using coolant.
When such a thermoelectric element is used for the thermoelectric generation, it is desired that the thermoelectric material has a high figure of merit (Z=3.times.10.sup.-3 [l/K] or more), and stably works under given use conditions for an extended time period. Further, it is necessary that the thermoelectric element is stable in an oxidizing atmosphere and its performance is not deteriorated.
Heretofore, as such thermoelectric materials, tellurium-based compounds such as Bi.sub.2 Te.sub.3, Bi.sub.2 Sb.sub.8 Te.sub.15, BiTe.sub.2 Se and PbTe, silicide-based compounds such as CrSi.sub.2, MnSi.sub.1.73, FeSi.sub.2, CoSi.sub.2, and their mixtures, and SiGe have been used.
However, although the thermoelectric materials composed of the tellurium-based compounds represented by Bi-Te series have relatively high figures of merit, Z, of about 3.times.10.sup.-3 [l/K] near room temperature, which are indicative of the thermoelectric performance, the performance is deteriorated at above 300.degree. C. Accordingly, use temperatures are largely limited. Further, since a volatile component such as tellurium or selenium is contained in the material composition, the melting point is low and the chemical stability is low. Further, since producing steps are complex, the performance is likely to vary due to variations in the composition.
As compared with the thermoelectric material composed of the tellurium-based compound, the silicide-based materials have chemical stability, and can be used in a temperature range of 300.degree. C. or more. However, the thermoelectric performance of the silicide-based materials is as small as 3.times.10.sup.4 [l/K] as the figure of merit at the maximum, which is lower than that of the tellurium-based compounds by one order or more. As to the silicide-based compounds, sufficient thermoelectric performance comparable to the tellurium-based compounds has not been obtained.
Consequently, it has been demanded to develop thermoelectric materials for use at high temperatures, which have sufficient thermal resistance at high temperatures of 300.degree. C. or more, are stable in the oxidizing atmosphere with their performance being unlikely to be deteriorated, and have excellent thermoelectric performance at high temperatures.